The present invention is concerned with a method of manufacturing low aspect ratio radial tires.
For purposes of describing and claiming the present invention, "aspect ratio" shall mean the ratio of the section height of a tire to the section width of the same tire. The section height of a tire is equal to (overall tire diameter--nominal rim diameter).div.2. The section width of a tire is its axial width at the widest part of the tire. Furthermore, for purposes of describing and claiming the present invention, a "low aspect ratio" shall mean an aspect ratio in the range of about 0.40 to about 0.70.
It is well known by those skilled in the tire design and manufacturing technologies that if the beads of an unvulcanized radial ply tire carcass, that has not been assembled with any tread reinforcing structure, are locked into position, as when mounted upon a rim, and a pressure exerting fluid is introduced into the inflation chamber thus formed, the radial ply carcass will tend to assume an equilibrium profile commonly referred to in the tire art as its natural shape profile or equilibrium profile. A radial section of a radial tire carcass that has expanded to assume its natural shape profile has a pronounced curvature in the portion of the carcass that normally underlies the tread portion of a tire that has been completely assembled. As used herein, "axial" and "axially" refer to the axis of rotation of a tire, and "radial" and "radially" refer to planes in which the axis of rotation of the tire lie. As used herein, "radial ply tire carcass" and "radial tire carcass" refer to any assembly of radial tire structural components, such as plies, beads, tread reinforcement structures, etc., that does not include a "tread", "tread portion" or "ground-engaging tread". However, once a "radial ply tire carcass", "radial tire carcass", or "carcass" has been assembled with or provided with a "tread", "tread material", "tread portion" or "ground-engaging tread", it will be referred to herein as a "radial tire", "completed radial tire", "radial ply tire", or simply as a "tire".
It is a common practice in the tire design and manufacturing technologies to have a radial ply tire carcass provided with a tread reinforcing structure to restrict the expansion of the carcass, and the completed radial tire, in the radial direction. Large radial tires in the size range of about 18.00-25 to about 36.00-51, often referred to as "earthmover tires" because of the large vehicles that they are designed to be mounted upon, may be maintained in the unvulcanized state mounted upon bladder assemblies having a minimal inflation pressure in the range of about 3 to 15 pounds per square inch (p.s.i.), (20,670 to 103,350 Pascales). Although the tread reinforcing structure restricts the diameter of an unvulcanized radial tire, even a minimal inflation pressure may cause the radial tire to expand to a larger than desired diameter during the time between the installation of the tread reinforcing structure around the radial carcass plies and the placing of the radial tire into a mold for vulcanization. The molds in which radial tires are vulcanized have metal tread rings with a fixed inside diameter, which determines the outside diameter of the ground-engaging portion of the tread of a completed radial tire. If the outside diameter of an unvulcanized radial tire has become too large, it will not fit into the mold for vulcanization. It should be noted that this problem could be encountered even in the manufacturing of small passenger car tires and similar articles, and that the method of manufacturing described and claimed herein is not restricted to the manufacture of radial ply earthmover tires.
The present invention is concerned with the use of a restrictor band, having restricting elements which are substantially parallel to each other, and form a substantially zero-degree orientation relative to the mid-circumferential plane of a radial tire or radial tire carcass. As used herein the "mid-circumferential plane" of a radial tire, or a radial tire carcass, shall mean a plane that is perpendicular to the tire or carcass' axis of rotation and located midway between the axial extent of a carcass reinforcing cord of the tire, or carcass, at its greatest axial width. The restrictor band restricts, but does not totally prevent, expansion of an unvulcanized radial tire carcass, or radial tire beyond a predetermined outside diameter at its mid-circumfential plane. The predetermined outside diameter is less than, or equal to, the inside diameter of the tread ring of a mold in which the tire is to be vulcanized.
While the use of zero-degree overlays in passenger car tires to improve high speed performance is known in the tire design and manufacturing technologies, it is believed that the use of restrictor band material to restrict the growth of an unvulcanized radial ply carcass or radial ply tire during the manufacturing process, such as to enable the manufacturing of low aspect ratio earthmover tires was heretofore unknown in the tire art.
There is provided in accordance with one aspect of the present invention a method of manufacturing a radial tire comprising the steps of: (a) providing an unvulcanized first-stage radial tire carcass; (b) expanding the first-stage radial tire carcass into a toroidal-like configuration; (c) assembling the first-stage radial tire carcass with a tread reinforcing structure, said tread reinforcing structure including at least two tread reinforcing plies and at least one circumferential wrap of substantially inextensible restrictor band material comprising restricting elements which are substantially parallel to each other such that said restricting elements form a substantially zero-degree orientation with respect to the mid-circumferential plane of said radial tire carcass, said restrictor band material thereby restricting expansion of the unvulcanized radial tire carcass beyond a predetermined outside diameter at its mid-circumferential plane; (d) applying tread material circumferentially about the radial tire carcass assembled in step (c) to form an unvulcanized radial tire; and (e) thereafter placing the unvulcanized radial tire into a curing mold, having a tread ring with an inside diameter sufficient to receive said unvulcanized tire, and applying heat and pressure to vulcanize said radial tire.
There is provided in accordance with another aspect of the invention a method of manufacturing a radial tire comprising the steps of: (a) providing an unvulcanized first-stage radial tire carcass; (b) providing a pre-assembled tread reinforcing structure that is attached by means for attachment to an inner circumferential surface of an annular ring, said tread reinforcing structure including at least two tread reinforcing plies and at least one circumferential wrap of substantially inextensible restrictor band material, said restrictor band material comprising restricting elements which are substantially parallel to each other such that said restricting elements form a substantially zero-degree orientation with respect to the mid-circumferential plane of said radial tire carcass, said restrictor band material thereby preventing expansion of the unvulcanized radial tire carcass beyond a predetermined outside diameter at its mid-circumferential plane; (c) mounting said first-stage radial tire carcass on a bladder assembly and introducing only enough pressure exerting fluid into said bladder assembly as is necessary to maintain said tire carcass in a fixed relationship with said bladder assembly; (d) inserting said bladder assembly, with said tire carcass mounted thereon, into said annular ring such that said pre-assembled tread reinforcing structure is axially centered with respect to said first-stage radial tire carcass, increasing the inflation pressure in said bladder assembly such that said first-stage radial tire carcass expands into a toroidal-like shape and becomes firmly adhered to said tread reinforcing structure; (e) removing said annular ring from around said tread reinforcing structure leaving said tread reinforcing structure assembled with said radial tire carcass; (f) applying tread material circumferentially about the unvulcanized radial tire carcass manufactured in step (e) to form an unvulcanized radial tire, and thereafter maintaining an internal inflation pressure in said bladder assembly that is sufficient to at least maintain said tire in a fixed relationship with said bladder assembly; and (g) thereafter placing the unvulcanized radial tire into a curing mold, having a tread ring with an inside diameter sufficient to receive said unvulcanized tire, and applying heat and pressure to vulcanize said radial tire.